Method for manufacturing actuator

ABSTRACT

A method for manufacturing an actuator includes: creating a semi-cured product in a semi-cure state in such a manner that an element material that is to be a string-shaped actuator element of all actuator that is made of dielectric elastomer and includes the actuator element is formed in a string shape and that the element material is heated to its cure temperature or higher at least one of in and after formation; and creating a fully-cured product with a cure reaction of the semi-cured product completed in such a manner that the semi-cured product is heated while elongated in its axial direction.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2007-297326 filed on Nov. 15, 2007 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing an actuator that includes an actuator element capable of outputting a driving force by expanding and contracting according to an applied voltage.

2. Description of the Related Art

An example of a hollow string-shaped actuator is described in Japanese Patent Application Publication No. JP-A-2003-230288. The disclosed actuator includes a hollow string-shaped actuator element and a pair of electrode layers disposed on the inner and outer surfaces of the actuator element.

As described in paragraph [0024] in Japanese Patent Application Publication No. JP-A-2003-230288, the disclosed actuator is manufactured in the following method: firstly, the electrode layer on the inner surface is formed by coating conductive material on the outer surface of a needle-shaped die; secondly, the actuator element is formed by coating dielectric elastomer on the outer surface of the electrode layer; thirdly, the electrode layer on the outer surface is formed by coating conductive material on the outer surface of the actuator element; and lastly, the actuator having a three-layer structure is obtained by extracting the needle-shaped die. The actuator disclosed in Japanese Patent Application Publication No. JP-A-2003-230288 is manufactured in the above manner.

The string-shaped actuator outputs a driving force by expanding and contracting in the axial direction. It is, therefore, required that the string-shaped actuator have high tensile strength in the axial direction. It is also required that the string-shaped actuator have high durability. However, it is difficult to manufacture an actuator with adequate tensile strength and durability by the method for manufacturing an actuator described in Japanese Patent Application Publication No. JP-A-2003-230288. Even though an actuator manufactured by the method for manufacturing an actuator described in Japanese Patent Application Publication No. JP-A-2003-230288 is elongated to secure expansion and contraction, it has little strength, showing small expansion due to voltage application.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention to provide a method for manufacturing an actuator that allows manufacturing an actuator with adequate tensile strength and durability.

(1) In order to solve the above problems, according to a first aspect of the present invention, a method for manufacturing an actuator includes: creating a semi-cured product in a semi-cure state in such a manner that an element material that is to be a string-shaped actuator element of an actuator that is made of dielectric elastomer and includes the actuator element is formed in a string shape and that the element material is heated to a cure temperature thereof or higher at least one of in and after formation; and the method also includes creating a fully-cured product with a cure reaction of the semi-cured product completed in such a manner that the semi-cured product is heated while elongated in an axial direction thereof. The “semi-cure state” means a state in which cure has yet to reach complete saturation.

In the process of creating a semi-cured product, a semi-cured product in a semi-cure state is created. In a process of creating a fully-cured product, a cure reaction is completed in such a manner that the semi-cured product is heated while elongated. This enables orientation of molecules constituting the fully-cured product in the axial direction. It is, therefore, possible to manufacture the actuator with adequate tensile strength and durability. By orienting the molecules constituting the fully-cured product in the axial direction, the dielectric constant is also improved, and a stronger Coulomb force is generated. This allows obtaining a larger degree of deformation of the actuator.

If an uncured product is elongated in the axial direction as it is, the element material in an uncure state only flows due to elongation, which does not facilitate orientation of its molecules. If elongation in the axial direction is carried out when a full-cure state is achieved, the dielectric elastomer in the full-cure state is only elastically deformed due to elongation, which does not facilitate orientation of its molecules, either. On the other hand, when elongation is carried out in the semi-cure state, the semi-cured product has proper flowability and viscoelasticity, which facilitates orientation of its molecules.

(2) According to a second aspect of the present invention, the method for manufacturing an actuator based on the above configuration (1) preferably has a configuration in which the process of creating a semi-cured product is forming the element material in a string shape by extrusion. The string-shaped actuator element may be used as a single actuator element. Multiple actuator elements that are bundled or knitted in the form of knitting may also be used. Therefore, the actuator element is required to have high dimensional accuracy. With the method for manufacturing an actuator described in Japanese Patent Application Publication No. JP-A-2003-230288, it is possible to increase the dimensional accuracy of the actuator. However, this method for manufacturing an actuator is not suitable for mass production.

On the other hand, with the method for manufacturing an actuator according to the present configuration, the semi-cured product is created by combining extrusion forming with heating properly in the process of creating a semi-cured product. Extrusion forming is suitable for mass production of elongated objects. The method for manufacturing an actuator according to the present configuration, therefore, allows mass production of the semi-cured product and thus of the actuator.

With the method for manufacturing an actuator according to the present configuration, it is possible to elongate the semi-cured product to adjust its form even when the semi-cured product cannot be formed into a desired shape only by extrusion. This makes it possible to increase the dimensional accuracy of the actuator. Thus, the method for manufacturing an actuator according to the present configuration allows mass production of the actuator with high dimensional accuracy.

(2-1) In the above configuration (2), it is preferable that the fully-cured product has a radial thickness in a range from 0.02 mm to 0.50 mm, inclusive, and has an external diameter in a range from 0.1 mm to 2.0 mm, inclusive. The “external diameter” means the diameter when the cross section in the radial direction is circular; and the maximum length in the radial direction when the cross section in the radial direction is polygonal.

Extrusion forming is normally disadvantageous for creating a product that is radially thin and small in diameter. However, the method for manufacturing an actuator according to the present configuration includes the process of creating a fully-cured product in which the semi-cured product is elongated in the axial direction. This facilitates creation of the actuator element that is radially thin and small in diameter even though the semi-cured product is created by extrusion forming. In other words, it is unnecessary to create in extrusion forming the semi-cured product that is radially thin and small in diameter, which reduces restrictions on the processes.

The radial thickness is set at 0.50 mm or smaller because, if the radial thickness exceeds 0.50 mm, the diameter of the actuator element and the applied voltage needed for operation become too large. On the other hand, the radial thickness is set at 0.02 mm or larger because, if the radial thickness is smaller than 0.02 mm, dielectric breakdowns more readily occur and the modulus of elasticity of the electrodes has more influence on the fully-cured product, which reduces its displacement due to voltage application.

The external diameter is set at 2.0 mm or smaller because, if the external diameter exceeds 2.0 mm, the diameter of the actuator element becomes too large. On the other hand, the diameter is set at 0.1 mm or larger because, if the diameter is smaller than 0.1 mm, it is difficult to secure an end of the actuator element.

(3) According to a third aspect of the present invention, the method for manufacturing an actuator based on the above configuration (2) preferably has a configuration in which the process of creating a semi-cured product includes forming and heating to create the semi-cured product by heating the element material to the cure temperature thereof or higher while forming the element material in a string shape by extrusion.

With the method for manufacturing an actuator according to the configuration, the element material can be semi-cured while it is formed, which reduces man-hours and allows heating the element material using heat generated in extrusion forming (although it is optional to use the heat).

(4) According to a fourth aspect of the present invention, the method for manufacturing an actuator based on the above configuration (2) preferably has a configuration in which the process of creating a semi-cured product includes extrusion-forming the element material in a string shape to create a formed product, and heating the formed product thus extrusion-formed to a cure temperature thereof or higher to create the semi-cured product in the semi-cure state.

In the method for manufacturing an actuator according to the present configuration, extrusion forming and heating are carried out in different processes. This provides a high degree of flexibility in making respective conditions for extrusion forming and heating, which facilitates extrusion forming and creation of the semi-cured product.

(5) According to a fifth aspect of the present invention, the method for manufacturing an actuator based on the above configuration (4) preferably has a configuration in which the actuator is formed of a number n (n≧1) of layers of actuator elements and the number n of electrode layers made of conductive elastomer that are alternately laminated in a radial direction of the actuator, and have a conductive fluid electrode in a liquid or gel form inside the actuator element that is an innermost layer. It is also preferable that the extrusion-forming process includes forming by hollow extrusion to create the formed product having a hollow axial part and a multilayer structure in such a manner that the number n of element materials and the number n of electrode materials that are to be the respective electrode layers are formed by multilayer extrusion; and it is also preferable that the method also includes injecting the fluid electrode into the axial part of the formed product between the process of forming by hollow extrusion and the heating process.

With the method for manufacturing an actuator according to the present configuration, it is possible to integrally form the actuator elements and electrode layers by multilayer extrusion. This reduces man-hours because the actuator elements and electrode layers need not be specially bonded. This also increases the strength of bonding between the actuator elements and electrode layers.

The actuator manufactured by the method for manufacturing an actuator according to the present configuration has a fluid electrode in its axial part. This reduces a possibility that the fluid electrode, in operation, may restrict expansion and contraction of the actuator element that is the innermost layer.

When an electrode layer is formed by applying it on an actuator element, the difficult part is an operation of applying the electrode layer on the inner surface of the actuator element that is the innermost layer. With the method for manufacturing an actuator according to the configuration, the fluid electrode is disposed inside the actuator element that is the innermost layer, which eliminates a need to apply an electrode layer on the inner surface of the actuator element that is the innermost layer.

(6) According to a sixth aspect of the present invention, the method for manufacturing an actuator based on the above configuration (2) preferably has a configuration in which the actuator is formed of a number n (n≧1) of layers of actuator elements and a number n+1 of electrode layers made of conductive elastomer that are alternately laminated in a radial direction of the actuator. It is also preferable that the process of creating a semi-cured product includes creating the semi-cured product having a multilayer structure in such a manner that the number n of element materials and the number n+1 of electrode materials that are to be the respective electrode layers are formed by multilayer extrusion and that the element materials and electrode materials are heated to cure temperatures thereof or higher at least one of in and after formation.

With the method for manufacturing an actuator according to the present configuration, it is possible to integrally form the actuator elements and electrode layers by multilayer extrusion. This reduces man-hours because the actuator elements and electrode layers need not be specially bonded. This also increases the strength of bonding between the actuator elements and electrode layers.

(7) According to a seventh aspect of the present invention, the method for manufacturing an actuator based on the above configuration (6) preferably has a configuration in which one of the electrode layers that is the innermost layer is a solid bar electrode having a bar shape. With the method for manufacturing an actuator according to the present configuration, it is possible to integrally form a solid actuator with a bar electrode in its axial part. The semi-cured product manufactured by the method for manufacturing an actuator according to the present configuration does not also easily crush when it is elongated, in comparison with the semi-cured product that is in the shape of a tube.

(8) According to an eighth aspect of the present invention, the method for manufacturing an actuator based on the above configuration (6) preferably has a configuration in which the actuator has a core rod made of dielectric elastomer inside one of the electrode layers that is the innermost layer. It is also preferable that the process of creating the semi-cured product having a multilayer structure includes creating the solid semi-cured product having a multilayer structure in such a manner that the number n of the element materials, the number n+1 of the electrode materials that are to be the respective electrode layers and a core rod material that is to be the core rod are formed by multilayer extrusion and that the element materials, electrode materials and core rod material are heated to cure temperatures thereof or higher at least one of in and after formation.

With the method for manufacturing an actuator according to the present configuration, it is possible to integrally form a solid actuator with an insulating core rod in its axial part. The semi-cured product manufactured by the method for manufacturing an actuator according to the present configuration does not also easily crush when it is elongated, in comparison with the semi-cured product that is in the shape of a tube.

(9) According to a ninth aspect of the present invention, the method for manufacturing an actuator based on the above configuration (1) preferably has a configuration in which the modulus of elasticity of the semi-cured product in the semi-cure state is improved by a percentage ranging from 20% to 60%, inclusive, in comparison with that of an uncured product when a difference between the modulus of elasticity of the fully-cured product and that of the uncured product in an uncure state prior to beginning of cure is deemed 100%.

An uncured product becomes a semi-cured product and fully-cured product due to a cure reaction. The improvement of the modulus of elasticity over the uncured product is set at 60% or lower because, if it exceeds 60%, the formed product easily regains its original shape and has a poor orientational property even though the semi-cured product is cured while elongated. On the other hand, the improvement of the modulus of elasticity over the uncured product is set at 20% or higher because, if it is lower than 20%, the orientation is prevented even though the formed product is elongated and it is easily cut.

(10) According to a tenth aspect of the present invention, the method for manufacturing an actuator based on the above configuration (1) preferably has a configuration in which the actuator has an electrode layer disposed on a surface of the actuator element. It is also preferable that the method also includes applying an electrode material that is to be the electrode layer on a surface of the fully-cured product after the process of creating a fully-cured product. With the present configuration, it is easy to dispose the electrode layer on the surface of the actuator element.

(11) According to an eleventh aspect of the present invention, the method for manufacturing an actuator based on the above configuration (1) preferably has a configuration in which the semi-cured product is elongated at an elongation rate of 45% or higher in the process of creating a fully-cured product.

The elongation rate R1 is given by an equation R1={(L1−L0)/L0}×100, when the length of the semi-cured product in the axial direction is represented by the length L0 when it is not elongated and by the length L1 when it is elongated. The elongation rate is set at 45% or higher because, if it is lower than 45%, the displacement of the actuator element corresponding to an applied voltage does not easily increase. It is more preferable that the elongation rate is set at 50% or higher, which gives a further increase in displacement of the actuator element corresponding to an applied voltage.

With the method for manufacturing an actuator according to some aspects of the present invention, it is possible to manufacture an actuator with adequate tensile strength and durability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an actuator manufactured by a manufacturing method that is an embodiment of the method for manufacturing an actuator according to the present invention;

FIG. 2 is an end view of the actuator;

FIG. 3A is a schematic view showing an extrusion forming process;

FIG. 3B is a schematic view showing a cutting process;

FIG. 3C is a schematic view showing a heating process;

FIG. 4 is a schematic view showing a process of creating a fully-cured product; and

FIG. 5 is a graph showing the displacement rate corresponding to the voltage applied to the samples used in examples and comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a method for manufacturing an actuator according to an embodiment of the present invention will be described.

Actuator Configuration

Hereinafter, a configuration of an actuator manufactured by a method for manufacturing an actuator according to the embodiment of the present invention will be described. FIG. 1 is a perspective view of the actuator manufactured by the method for manufacturing an actuator according to the embodiment. Part of the configuration is transparently shown for convenience of description. FIG. 2 is an end view of the actuator. As shown in FIGS. 1 and 2, an actuator 1 includes an actuator element 20 and a pair of inner and outer electrode layers 21 and 22.

The actuator element 20 is made of nitrile rubber and has the shape of a string. Nitrile rubber molecules constituting the actuator element 20 are oriented in the axial direction. The inner electrode layer 21 and outer electrode layer 22 are made of acrylic binder compounded with carbon. A radial thickness d1 of the actuator element 20 (corresponding to the fully-cured product described in (2-1)) is 0.2 mm. The external diameter d2 is 1.5 mm.

With a voltage applied across the electrode layers 21 and 22, an electrostatic attraction between the electrode layers 21 and 22 increases; and the actuator element 20 contracts in the radial direction and expands in the axial direction between the electrode layers 21 and 22. With the voltage application stopped, the actuator element 20 expands in the radial direction and contracts in the axial direction by its elastic restitution force. Thus, the actuator element 20 expands and contracts in the axial direction in response to an application of a voltage or an end of the application. A driving force is derived from the actuator 1 by using the expansive and contractive deformation.

Method for Manufacturing Actuator

Hereinafter, a method for manufacturing the actuator 1 according to the embodiment will be described. The method for manufacturing an actuator according to the embodiment includes a process of creating a multilayer semi-cured product and a process of creating a fully-cured product.

Firstly, the process of creating a multilayer semi-cured product will be described. The process of creating a multilayer semi-cured product includes an extrusion forming process, cutting process and heating process. FIGS. 3A, 3B and 3C are schematic views of the process of creating a multilayer semi-cured product. FIGS. 3A, 3B and 3C show the extrusion forming process, cutting process and heating process, respectively.

In the extrusion forming process, materials are introduced into material feeders 90 to 92 of an extruder 9, as shown in FIG. 3A. More specifically, an element material 20 a that is to be the actuator element 20 shown in FIG. 2 is introduced into the material feeder 90; an electrode material 21 a that is to be the inner electrode layer 21 shown in FIG. 2 is introduced into the material feeder 91; and an electrode material 22 a that is to be the outer electrode layer 22 shown in FIG. 2 is introduced into the material feeder 92. A tube-shaped formed product 1 a is extruded from dies 93. The formed product 1 a has a three-layer structure with the electrode material 21 a, element material 20 a and electrode material 22 a layered in this order from inside to outside in the radial direction. In the cutting process, the formed product 1 a is cut into predetermined lengths L in the axial direction, as shown in FIG. 3B.

In the heating process, the formed product 1 a is transferred into a furnace body 80 of a curing furnace 8, as shown in FIG. 3C. More specifically, the formed product 1 a is extended between upstream rollers 81 and downstream rollers 82 that are both stationary. The formed product 1 a is heated according to predetermined temperature patterns so as to make a semi-cured product 1 b in a semi-cure state.

When the difference between the modulus of elasticity of the actuator element 20 and that of an uncured product (the element material 20 a included in the formed product 1 a) in an uncure state prior to beginning of cure is deemed 100%, the modulus of elasticity of the semi-cured product 1 b is improved by a percentage ranging from 20% to 60%, inclusive, in comparison with that of the uncured product.

The modulus of elasticity of the semi-cured product 1 b is 0.73 MPa; that of the uncured product is 0.6 MPa; and that of the actuator element 20 is 0.86 MPa.

The modulus of elasticity of the semi-cured product 1 b, uncured product or fully-cured product (actuator element 20) is measured by evaluation of tensile strength. More specifically, the actuator element 20 (60 mm long in the axial direction) having a 20-mm reference line on the center is stretched at a speed of 100 mm/min with the upper end (one end in the axial direction) and the lower end (the other end in the axial direction) attached to a tensile strength test apparatus. A stress with the reference line elongated by 50% is evaluated.

Secondly, the process of creating a fully-cured product will be described. FIG. 4 is a schematic view of the process of creating a fully-cured product. In the process of creating a fully-cured product, the downstream rollers 82 are moved with the upstream rollers 81 stationary, as shown in FIG. 4. The semi-cured product 1 b is elongated in the axial direction. While elongated, the semi-cured product 1 b is heated according to predetermined temperature patterns so as to make a fully-cured product 1 c in a full-cure state.

Lastly, the fully-cured product 1 c is cut in a predetermined length in the axial direction, making the actuator 1 shown in FIG. 2. The actuator 1 according to the embodiment is thus manufactured.

ADVANTAGEOUS EFFECTS

Advantageous effects brought by the method for manufacturing the actuator 1 according to the embodiment will be described hereinafter. In the method for manufacturing the actuator 1 according to the embodiment, the cure reaction is completed in such a manner that the semi-cured product 1 b is heated while elongated in the process of creating a fully-cured product. This allows orientation of nitrile rubber molecules constituting the actuator element 20 in the axial direction. It is, therefore, possible to manufacture the actuator 1 with adequate tensile strength and durability.

If the formed product 1 a is elongated in the axial direction as it is, the element material 20 a in the uncure state only flows due to elongation, which does not facilitate orientation of its molecules. If elongation in the axial direction is carried out when the full-cure state is achieved, the nitrile rubber in the full-cure state is only elastically deformed due to elongation, which does not facilitate orientation of its molecules, either. On the other hand, when elongation is carried out in the semi-cure state, the semi-cured product 1 b has proper flowability and viscoelasticity, which facilitates orientation of its molecules.

In the method for manufacturing the actuator 1 according to the embodiment, the semi-cured product 1 b is created using the extruder 9. Extrusion forming is suitable for mass production of elongated objects. The method for manufacturing the actuator 1 according to the embodiment, therefore, allows mass production of the semi-cured product 1 b and thus of the actuator 1.

With the method for manufacturing the actuator 1 according to the embodiment, it is possible to elongate the semi-cured product 1 b to adjust its form when the semi-cured product 1 b cannot be formed into a desired shape only by extrusion. This makes it possible to increase the dimensional accuracy of the actuator 1. Thus, the method for manufacturing the actuator 1 according to the embodiment allows mass production of the actuator 1 with high dimensional accuracy.

The radial thickness d1 of the actuator element 20 included in the actuator 1 according to the embodiment is 0.2 mm. The external diameter d2 is 1.5 mm. Extrusion forming is normally disadvantageous for creating the formed product 1 a that is radially thin and small in diameter. The method for manufacturing the actuator 1 according to the embodiment includes the process of creating a fully-cured product in which the semi-cured product 1 b is elongated in the axial direction. This facilitates creation of the actuator element 20 that is radially thin and small in diameter even though the semi-cured product 1 b is created by extrusion forming. In other words, it is unnecessary to create in extrusion forming the semi-cured product 1 b that is radially thin and small in diameter, which reduces restrictions on the processes. This allows first creating the semi-cured product 1 b larger than desired in diameter by extrusion forming and then creating the fully-cured product 1 c with a desired diameter by elongation.

In the method for manufacturing the actuator 1 according to the embodiment, the process of creating a multilayer semi-cured product includes the extrusion forming process, cutting process and heating process, as shown in FIGS. 3A, 3B and 3C; that is, extrusion forming and heating are carried out in different processes. This provides a high degree of flexibility in making respective conditions for extrusion forming and heating, thereby facilitating extrusion forming and creation of the semi-cured product 1 b.

With the method for manufacturing the actuator 1 according to the embodiment, it is possible to integrally form the actuator element 20 and electrode layers 21 and 22 by multilayer extrusion. This reduces man-hours because the actuator element 20 and electrode layers 21 and 22 need not be specially bonded. This also increases the strength of bonding between the actuator element 20 and the respective electrode layers 21 and 22.

With the method for manufacturing an actuator according to the embodiment, the modulus of elasticity of the semi-cured product 1 b is improved by a percentage ranging from 20% to 60%, inclusive, in comparison with that of the uncured product when the difference between the modulus of elasticity of the actuator element 20 and that of the uncured product in the uncure state prior to beginning of cure is deemed 100%. Even though the semi-cured product 1 b is cured while elongated, therefore, it does not easily regain its former shape; it has a better orientation of molecules; and it is not easily cut.

The embodiment of the method for manufacturing an actuator according to an aspect of the present invention has been described above. Embodiments of the present invention, however, are not particularly limited to the above embodiment. Various embodiments that can be modified or improved by those skilled in the art may be utilized.

In the above embodiment, for example, the formed product 1 a is processed into the semi-cured product 1 b and fully-cured product 1 c after being cut in a predetermined length L in the axial direction; however, the formed product 1 a that is in elongated shape may be processed in such a manner that the formed product 1 a is processed continuously into the semi-cured product 1 b and fully-cured product 1 c without being cut. In this situation, a curing furnace for semi-cure and a curing furnace for full cure may be arranged upstream and downstream of the production line, respectively. Transfer of the formed product 1 a, semi-cured product 1 b and fully-cured product 1 c may be carried out on belt conveyors, rollers or the like. Elongation of the semi-cured product 1 b may be carried out, for example, by making the downstream transfer speed faster than the upstream transfer speed on both sides of the curing furnace for full cure in the transfer direction. The created fully-cured product 1 c may be reeled in, for example, on a bobbin for storage.

In the process of creating a fully-cured product of the above embodiment, the semi-cured product 1 b is elongated by moving only the downstream rollers 82 with the upstream rollers 81 stationary, as shown in FIG. 4. However, the semi-cured product 1 b may be elongated by rotating the downstream rollers 82 with the upstream rollers 81 stationary. The semi-cured product 1 b may also be elongated by rotating the upstream rollers 81 and downstream rollers 82 in the opposite directions. The semi-cured product 1 b may also be gripped from the outside in the radial direction, for example, with clamping rings instead of the upstream rollers 81 and downstream rollers 82 for elongation of the semi-cured product 1 b.

In the heating process under the above embodiment, the formed product 1 a in the uncure state is extended between the stationary upstream rollers 81 and downstream rollers 82, as shown in FIG. 3C; however, it may be placed, for example, on a conveyor belt. In this situation, the formed product 1 a is not easily deformed under its weight.

In the above embodiment, extrusion formation and heating are carried out separately, as shown in FIGS. 3A, 3B and 3C; however, it may be carried out simultaneously. In this situation, the formed product 1 a may be made into the semi-cured product 1 b using heat generated in extrusion forming. This makes it possible to create the semi-cured product 1 b without using the curing furnace 8.

In the above embodiment, the actuator 1 is created so as to be hollow, as shown in FIG. 2; however, it may be created so as to be solid. More specifically, the electrode layer 21 that is the innermost layer may be a bar electrode. In this case, an electrode material for the bar electrode may be introduced into the material feeder 91. A core rod made of dielectric elastomer may also be disposed inside the electrode layer 21 that is the innermost layer. In this case, another extruder may be disposed so that a core rod material for the core rod may be introduced into a material feeder of the extruder.

Instead of the electrode layer 21 that is the innermost layer, a fluid electrode in a liquid or gel form may be disposed. The fluid electrode may be injected into the formed product 1 a in an injection process that is provided between the cutting process (shown in FIG. 3B) and the heating process (shown in FIG. 3C). In this case, the fluid electrode may be injected into the formed product 1 a in such a manner that: one end in the axial direction of the formed product 1 a that has been cut in a predetermined length L in the axial direction is sealed first; the fluid electrode is injected from the other end in the axial direction into the formed product 1 a next; and the other end in the axial direction is sealed lastly. Alternatively, the fluid electrode may be injected into the formed product 1 a in such a manner that: both the ends of the formed product 1 a in the axial direction are sealed first, and then the fluid electrode is injected into the sealed space with a syringe. It is preferable that the fluid electrode has a small coefficient of thermal expansion, which prevents deformation in the heating process. It is also preferable that the fluid electrode does not readily vaporize in the heating process and the process of creating a fully-cured product, which prevents the fluid electrode from vaporizing and leaking from the formed product 1 a, semi-cured product 1 b or fully-cured product 1 c.

In the above embodiment, the whole actuator 1 is integrally manufactured by extrusion forming; however, only the actuator element 20 may be created by extrusion forming; and the electrode layers 21 and 22 may be disposed on the inner and outer surfaces of the actuator element 20 by coating or dipping. This simplifies the structure of the extruder 9, which reduces costs concerning extrusion forming.

In the above embodiment, the material of the actuator element 20 is nitrile rubber; however, the material of the actuator element 20 is not particularly limited provided that it deforms according to the electrostatic attraction between the electrode layers 21 and 22. Examples of dielectric elastomer with a high dielectric property and dielectric breakdown strength include the above nitrile rubber that is a acrylonitrile-butadiene copolymer rubber, a nitrite rubber that is the hydrogenated acrylonitrile-butadiene copolymer rubber which added hydrogen to the acrylonitrile-butadiene copolymer rubber, a silicone rubber, a fluororubber and a polyurethane rubber.

The shape of the cross section of the actuator 1 at right angles to the axis is not also particularly limited. The shape may be a perfect circle (which includes a perfect circular ring; the same shall apply hereinafter.), an ellipse or a flat oval (a shape that is a pair of semicircles that face one another and are connected with a pair of straight lines). The shape may also be polygonal, such as triangular, rectangular and hexagonal.

The radial thickness d1 and external diameter d2 (shown in FIG. 2) of the actuator element 20 are not also particularly limited. They may be properly determined according to the use of the actuator 1 or the like. From a viewpoint of downsizing, lowering of driving voltage, an increase in displacement of the actuator 1, for example, it is preferable that the radial thickness d1 of the actuator element 20 be smaller. In this situation, the radial thickness d1 of the actuator element 20 should be in a range from 5 μm to 480 μm, inclusive, its dielectric breakdown strength and other properties taken into consideration. More preferably, it should be in a range from 10 μm to 200 μm, inclusive.

Under the above embodiment, the material of the electrode layers 21 and 22 is acrylic binder compounded with carbon; however, it is not particularly limited. It is preferable that the material of the electrode layers 21 and 22 is expandable and contractible with expansion and contraction of the actuator element 20. When the electrode layers 21 and 22 expand and contract with the actuator element 20, the electrode layers 21 and 22 do not readily prevent deformation of the actuator element 20, which facilitates acquisition of desired displacements. The electrode layers 21 and 22 may be formed, for example, by applying a paste or paint compounded with oil or elastomer used as a binder onto a conductive material that is made of a carbon material, such as carbon black and carbon nanotubes. A flexible material, such as a silicone rubber, acrylic rubber, ethylene-propylene-diene monomer terpolymer (EPDM), natural rubber (NR), isobutylene-isoprene (butyl) rubber (IIR), isoprene rubber (IR), acrylonitrile-butadiene copolymer rubber (NBR), hydrogenated nitrile rubber (H-NBR), Hydrin rubber, chloroprene rubber (CR) and polyurethane rubber, is suitable as an elastomer serving as a binder. For improvement in expandability and contractibility of the actuator element 20, the electrode layers 21 and 22 may be formed by attaching a finely-powdered conductive material, such as carbon black and carbon nanotubes, directly to the surfaces of the dielectric layer.

Another insulation layer may be laminated on the outer surface of the electrode layer 22 under the above embodiment. The material of the insulation layer is not particularly limited if it is capable of preventing current leakage from the electrode layer 22. For example, it is preferable that the material of the insulation layer, same as that of the electrode layers 21 and 22, is expandable and contractible with expansion and contraction of the actuator element 20. A flexible material, such as a silicone rubber, acrylic rubber, ethylene-propylene-diene monomer terpolymer (EPDM), natural rubber (NR), isobutylene-isoprene (butyl) rubber (IIR), isoprene rubber (IR), acrylonitrile-butadiene copolymer rubber (NBR), hydrogenated nitrile rubber (H-NBR), Hydrin rubber, chloroprene rubber (CR) and polyurethane rubber, is suitable as a material of the insulation layer. When the insulation layer and actuator element 20 are made of the same material, a greater driving force can be obtained.

The number of layers of the actuator element 20 and electrode layers 21 and 22 in the radial direction is not also particularly limited. A larger number of layers increase the driving force of the actuator 1 at the same applied voltage. The number of layers may be properly determined according to an applied voltage or desired driving force.

The actuator element 20 may be used as a single actuator element. Multiple actuator elements that are bundled may also be used. The actuator elements 20 that are bundled are capable of outputting a greater driving force and useful as an artificial muscle or the like.

Multiple actuator elements 20 are in the state knit by stockinet etc., and may be used. Multiple actuator elements 20 are in the state woven by plain fabric etc., and may be used. Bundles of multiple actuator elements 20 are may also be knitted or woven in a similar manner and used.

EXAMPLES

An experiment on actuators manufactured by the method for manufacturing an actuator according to the present invention will be described hereinafter.

Dimensions of Samples Used in Examples

The actuators used in examples 1 and 2 had the same configuration as that of the actuator according to the above embodiment (shown in FIGS. 1 and 2). The radial thickness of the actuator elements included in the actuators used in examples 1 and 2 was 0.2 mm. The external diameter was 1.5 mm. The internal diameter was 1.1 mm. The length in the axial direction was 10 mm.

Method for Manufacturing Samples Used in Examples

The method for manufacturing the actuators used in examples 1 and 2 included a process of creating a semi-cured product, a process of creating a fully-cured product and a process of applying electrodes. In the process of creating a semi-cured product, the semi-cured products in the semi-cure state were created in such a manner that the element material that is to be the actuator elements was formed into the shape of a string and was heated at 170 C for 1 minute. In the process of creating a fully-cured product, the fully-cured products were created with the cure reaction of the semi-cured products completed in such a manner that the semi-cured products were heated at 170 C for 14 minutes while elongated at predetermined elongation rates in the axial direction. The elongation rate for the actuator element used in example 1 was 50%. The elongation rate for the actuator element used in example 2 was 100%. In the process of applying an electrode material, the electrodes were disposed by applying and drying a solution containing conductive carbon dispersed in acrylic elastomer on the inner and outer surfaces of the respective actuator elements used in examples 1 and 2. After the processes, the actuator elements were hung from a ceiling at one end (the upper end) in the axial direction; and a 20-g weight was hung from the other end (lower end) of the actuator elements in the axial direction. The actuators used in examples 1 and 2 were thus created.

Dimensions of and Method for Manufacturing Sample Used in Comparative Example

The dimensions of an actuator used in a comparative example was the same as those of the actuators used in examples 1 and 2. The method for manufacturing the actuator used in this comparative example was the same as that for manufacturing the actuators used in examples 1 and 2 except that the semi-cured product was not elongated (or was elongated at the elongation rate of 0%) in the process of creating a fully-cured product.

Experimental Results

FIG. 5 shows the displacement rate corresponding to the voltage applied to the samples used in the examples and comparative example. In FIG. 5, the letter a is a given positive number. When the length in the axial direction of the samples used in the examples and comparative example is represented by the length X0 before voltage application and by the length X1 after voltage application, the displacement rate L2 is given by an equation R2={(X1−X0)/X0}×100. The applied voltage is a value per unit of radial thickness (1 μm) of the actuator element.

As shown in FIG. 5, the displacement rate corresponding to the applied voltage was largest in example 2 (in which the elongation rate was 100%), second-largest in example 1 (in which the elongation rate was 50%), and smallest in the comparative example (in which the elongation rate was 0%). The results show that the displacement rate with respect to a predetermined applied voltage increases as the elongation rate increases. The results also show that the difference A1 between the displacement rates of the comparative example and example 1 and the difference A2 between the displacement rates of example 1 and example 2 increase as the applied voltage increases.

The results also show that the applied voltage required to obtain a predetermined displacement rate decreases as the elongation rate increases. The results also show that the difference B1 between the voltages applied in the comparative example and example 1 and the difference B2 between the voltages applied in example 1 and example 2 increase as the displacement rate increases. 

1. A method for manufacturing an actuator, comprising: creating a semi-cured product in a semi-cure state in such a manner that an element material to be a string-shaped actuator element of an actuator that is made of dielectric elastomer and includes the actuator element is formed in a string shape and that the element material is heated to a cure temperature thereof or higher at least one of in and after formation; and creating a fully-cured product with a cure reaction of the semi-cured product completed in such a manner that the semi-cured product is heated while elongated in an axial direction thereof.
 2. The method for manufacturing an actuator according to claim 1, wherein the element material is formed in a string shape by extrusion in the process of creating a semi-cured product.
 3. The method for manufacturing an actuator according to claim 2, wherein the process of creating a semi-cured product includes forming and heating to create the semi-cured product by heating the element material to the cure temperature thereof or higher while forming the element material in a string shape by extrusion.
 4. The method for manufacturing an actuator according to claim 2, wherein the process of creating a semi-cured product includes extrusion-forming the element material in a string shape to create a formed product, and heating the formed product thus extrusion-formed to a cure temperature thereof or higher to create the semi-cured product in the semi-cure state.
 5. The method for manufacturing an actuator according to claim 4, wherein the actuator is formed of a number n (n≧1) of layers of actuator elements and the number n of electrode layers made of conductive elastomer that are alternately laminated in a radial direction of the actuator, and has a conductive fluid electrode in a liquid or gel form inside the actuator element that is an innermost layer, the extrusion-forming process includes forming by hollow extrusion to create the formed product having a hollow axial part and a multilayer structure in such a manner that the number n of element materials and the number n of electrode materials that are to be the respective electrode layers are formed by multilayer extrusion, and the method further comprises injecting the fluid electrode into the axial part of the formed product between the process of forming by hollow extrusion and the heating process.
 6. The method for manufacturing an actuator according to claim 2, wherein the actuator is formed of a number n (n≧1) of layers of actuator elements and a number n+1 of electrode layers made of conductive elastomer that are alternately laminated in a radial direction of the actuator, and the process of creating a semi-cured product includes creating the semi-cured product having a multilayer structure in such a manner that the number n of element materials and the number n+1 of electrode materials that are to be the respective electrode layers are formed by multilayer extrusion and that the element materials and the electrode materials are heated to cure temperatures thereof or higher at least one of in and after formation.
 7. The method for manufacturing an actuator according to claim 6, wherein one of the electrode layers that is the innermost layer is a solid bar electrode having a bar shape.
 8. The method for manufacturing an actuator according to claim 6, wherein the actuator has a core rod made of dielectric elastomer inside one of the electrode layers that is the innermost layer, and the process of creating the semi-cured product having a multilayer structure includes creating the solid semi-cured product having a multilayer structure in such a manner that the number n of the element materials, the number n+1 of the electrode materials that are to be the respective electrode layers and a core rod material that is to be the core rod are formed by multilayer extrusion and that the element materials, the electrode materials and the core rod material are heated to cure temperatures thereof or higher in or after, or in and after formation.
 9. The method for manufacturing an actuator according to claim 1, wherein a modulus of elasticity of the semi-cured product in the semi-cure state is improved by a percentage ranging from 20% to 60%, inclusive, in comparison with that of an uncured product when a difference between the modulus of elasticity of the fully-cured product and that of the uncured product in an uncure state prior to beginning of cure is deemed 100%.
 10. The method for manufacturing an actuator according to claim 1, wherein the actuator has an electrode layer disposed on a surface of the actuator element, and the method further comprises applying an electrode material that is to be the electrode layer on a surface of the fully-cured product after the process of creating a fully-cured product.
 11. The method for manufacturing an actuator according to claim 1, wherein the semi-cured product is elongated at an elongation rate of 45% or higher in the process of creating a fully-cured product.
 12. The method for manufacturing an actuator according to claim 2, wherein the fully-cured product has a radial thickness in a range from 0.02 mm to 0.50 mm, inclusive, and an external diameter in a range from 0.1 mm to 2.0 mm, inclusive. 